Fabrication, accelerometers

The advantages of miniaturization are now being exploited in areas beyond microelectronics. Adaptation of materials and processes originally devised for semiconductor manufacture has allowed fabrication of sensors (for example, pressure meters and accelerometers used in the automotive industry) (6,7), complex optical (8) and micromechanical (6,7,9) assembHes, and devices for medical diagnostics (6,7,10) using Hthographic resists. 

L. M. Roylance and J. B. Angell, A batch-fabricated silicon accelerometer, IEEE Trans. Electron Devices 26(12), 1911, 1979. 

An example of the practical consequences of vertical deflection is shown in Figure 5.5.2 for a comb-type accelerometer fabricated with polysilicon. In Fig. 5.5.2a, a positive stress gradient leads to an upward deflection of the fixed outer beams and a downward deflection of the proof mass. In Fig. 5.5.2b, the situation is reversed due to a negative stress gradient in the material [11]. 

In addition to the desired dependence on AC, it has a matching-dependent offset and gain that depends on parasitics. Any deviation of the reference capacitor Cref from the nominal value of the sense capacitance Cs appears as offset. Since in many applications AC is much smaller than C0, this offset often exceeds the signal. Offset cancellation should therefore occur early to minimize the dynamic range of the readout electronics. Care should also be taken for the trimming not to introduce a poor temperature coefficient. One solution fabricates the reference with the same process and in close proximity to the sense capacitor. The z axis accelerometer shown in Fig. 6.1.3 [7] utilizes two rnicromachined structures for the sense and reference. The suspension of the reference structure has been made intentionally stiff. 

FIGURE 2.4 Example of a silicon chip accelerometer fabricated using MEMS technology. The lower figure shows the upward deflection of the seismic mass with a downward acceleration. Typical dimensions of the silicon chip are 1 and 2 mm length and width, respectively and less than a mm thick. This means that the packaged chip can be very small. 

The most widely studied ceramics for MEMS applications are the PZTs because of their high k and high k. Thin films of PZT have been used in the fabrication of a range of different MEMS and can be integrated with silicon IC processing methods. Figure 31.23 illustrates some of the process steps used to fabricate a cantilever beam microsensor such as an accelerometer. The actual processing sequence requires over 50 individual steps. 

In the past decade, an increasing number of mHealth systems appeared in the form of clothing. For example, the LifeShirt from VivoMetrics is embedded with inductive plethys-mographic sensors, accelerometer, and ECG electrodes [17], Data are recorded in a small belt-worn PDA where they are stored or sent to a care center via mobile network. Another group integrated biosensors in the fabric of a shirt and developed their prototype which contains dry ECG electrodes, shock/fall sensor, breath rate sensor, temperature sensors, global positioning system (GPS) receiver, and wireless module [18]. 

Rittersma ZM, Splinter A, Bodecker A, Benecke W (2000) A novel surface-micromachined capacitive porous silicon humidity sensor. Sens Actuators B 68 210-217 Sim J-H, Cho C-S, Kim J-S, Lee J-H, Lee J-H (1998) Eight beam piezoresistive accelerometer fabricated by using a selective porous silicon etching method. Sens Actuators A 66 273-278 Steiner P, Lang W (1995) Micromachining applications of porous silicon. Thin Solid Films 255 52-58... 

In the following, the fabrication and characterization of micromachined high frequency focused polymer ultrasonic transducers in a manner that is compatible with CMOS microelectronics, and MEMS batch fabrication techniques, are described. The specifics of the electronics are not described here, but the interested reader may find more details elsewhere [75, 76, 81-84]. The transducer is capable of being manufactured on silicon wafers after the completion of CMOS electronics. These two key elements enable the eventual creation of a monolithic transducer chip that does not require modification of the standard circuit fabrication process. This type of transducer chip will likely follow the path of other MEMS devices such as accelerometers, gene chips and digital micromirror arrays, where batch production, high yields and... 

The heart rate can be measured by an ECG, and the posture and activity level can be measured by a three-axis accelerometer. In designing heart sensors, good contact with the skin is es sential, and interference with the working environment should be as minimum as possible. The electrodes for measuring heart rate could be textile electrodes based on electrical conductive stainless steel yams integrated into an iimer garment. The electrodes can be knitted or woven in a double-face fabric so that the outer part of the electrodes does not contain any conductive yam and insulates the electrodes from the outside environment (Sahin et al., 2005). 

Shahinpoor [930], working at the "Artificial Muscles Research Institute", University of New Mexico, Albuquerque, NM, USA, fabricated devices for a wide variety of applications based on electrochemomechanical principles, from ion conducting polymers (not CPs). These polymers included poly(acrylic acid-bisacrylamide) (PAAM), poly(2-acrylamido-2-methylpropanesulfonic acid (Poly(AMPS)), and polyacrylonitrile (PAN). While these are not CPs, Shahinpoor also indicated that similar action could be expected, with minor modifications, from CPs such as poly (ary lene vinylenes) and poly(thienylene vinylenes). Shahinpoor typically used a metal (e.g. Pt) + ion conductive polymer composite in place of the customary bilayers. Some of the applications envisioned, or demonstrated for ion conductive polymers, included microactuators, motion sensors, accelerometers, oscillating artificial muscles, inchworms, cardiac>circulation assistants, noiseless propulsion swimming robots for military applications, fully constituted contractile artificial muscles, miniature flying machines, and electrically controllable adaptive optical lenses (Fig. 21-51. The potential military applications of these have fueled much interest recently [931]. 

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